Caecilians, a worm-like underground amphibian, evolved unique ears to sense underground vibrations, according to a Carleton University Canada study. Researchers said that caecilians have an organ in the ear that is larger than that in other amphibians (frogs) and salamanders.

Dr Hillary Maddin, a lead author on the study, said that soil-borne animals experience same pressures as those flying or swimming. As a result, researchers predicted similar-shaped ears. But, caecilians have a larger ventral organ, probably used for transmitting vibrations from the ground to the ear.

Maddin said that underground animals have their heads on the ground all the time due to being limbless. Their ears are also non-functioning and therefore, cannot hear any airborne sounds. However, this larger organ determines vibrations and helps them track preys or know if predators are around.

"It's unique to this group," Maddin said in a statement.        

For the study, researchers examined salamanders, frogs and caecilians using a CT scanner. They built similar 3D digital replicas of each using digital images of the animals' ears. When researchers were measuring any variations between inner canals and shapes of the ears, they spotted the unusual ventral organ in every caecilian specimen.

"This is the first time this organ has been quantified - people have noted it before but it's never been shown that it's truly distinct in all members of the group. Even primitive species all have a larger vibration sensing organ," said Maddin.

Maddin said that this large ventral organ is not observed in other subterranean animals. This shows that despite facing same environmental problems, underground animals like molerats and caecillians evolved different coping mechanisms.

"Caecilians seem to just get in a hole and sit there, whereas an animal like a molerat keeps moving. This might explain why all the senses in the caecilians seem to be degenerating, while molerats utilise all of their senses to get information on their surroundings," said Maddin. "We know in the lab they're quite sedentary, so even though they have the ability to exploit the 3D environment, they may not actually be doing that."

The finding is published in the Journal of Anatomy.